The invention relates to a projection system and a light-guiding assembly thereof, and in particular to a projection system and a light-guiding assembly having a lens capable of decreasing light leakage and increasing the contrast of a projected image.
F-number
  (            f      /      #        ≡          f      D        )represents the ability of light from an optical element to converge, i.e., the ratio of focus length (f) and diameter of incident pupil (D). In the formula of
      θ    ≅                  Sin                  -          1                    ⁢              1                  2          ⁢          F                      ,a convergent angle is in proportion to f-number. That is to say, the smaller of the convergent angle, the more light beam can converge and the slower the light path.
FIG. 1 is a schematic view of a conventional Digital Light Processing (DLP) system. A light beam 100a generated from a light source 10a is guided and output by a light pipe 12a. The light beam 100a, before entering the light pipe 12a and exiting therefrom, maintains the same f-number. The output light beam 100a is sequentially projected from a projecting lens 14a to a reflecting lens 16a, reflected from the reflecting lens 16a to a power mirror 18a, reflected from the power mirror 18a to a light valve 20a (e.g. Digital Micro-mirror Device, DMD). Finally, an image reflected from light valve 20a the light valve 20a can be projected to a screen by a projecting lens 22a. 
The light valve 20a, a main device for the DMD projection system, comprises a ceramic substrate 19, a DMD chip 21 formed by a plurality of micromirrors, an aperture 22 and a window assembly 23. The window assembly 23 disposed on the ceramic substrate 19 is used to isolate the ceramic substrate 19, and the aperture 22 is disposed between the ceramic substrate 19 and the window assembly 23.
FIG. 4A is a schematic view showing distribution of light path of the conventional projection system of FIG. 1. The light beam is transmitted to the light valve 20a and reflected by the projecting lens 22a. A projected area by the light beam reflected from the projecting lens 22a is larger than the area of the DMD chip 21 if the reflected light beam is divergent. Scattered light formed by a light reflected by the window assembly 23, the aperture 22 and the ceramic substrate 19 enter the light processing system, cause light leakage and decrease the contrast of the projected image.
To adjust the contrast of the projection system, the number of lens assemblies must be added in the DLP system. However, the layout of the DLP system becomes complicated and the size of the DLP system must be increased.